Typically, each battery powered device such as a wireless communication device such as a cell-phone requires its own charger and power source, which is usually the AC power outlet. This becomes unwieldy when many devices need charging.
Approaches are being developed that use over the air power transmission between a transmitter and the device to be charged. These generally fall into two categories. One is based on the coupling of plane wave radiation (also called far-field radiation) between a transmit antenna and receive antenna on the device to be charged which collects the radiated power and rectifies it for charging the battery. Antennas are generally of resonant length in order to improve the coupling efficiency. This approach suffers from the fact that the power coupling falls off quickly with distance between the antennas. So charging over reasonable distances (e.g., >1-2m) becomes difficult. Additionally, since the system radiates plane waves, unintentional radiation can interfere with other systems if not properly controlled through filtering.
Other approaches are based on inductive coupling between a transmit antenna embedded, for example, in a “charging” mat or surface and a receive antenna plus rectifying circuit embedded in the host device to be charged. This approach has the disadvantage that the spacing between transmit and receive antennas must be very close (e.g. mms). Though this approach does have the capability to simultaneously charge multiple devices in the same area, this area is typically small, hence the user must locate the devices to a specific area. Therefore, there is a need to provide a wireless charging arrangement that accommodates flexible placement and orientation of transmit and receive antennas.
Efficiency is of importance in a wireless power transfer system due to the losses occurring in the course of wireless transmission of power. Since wireless power transmission if often less efficient that wired transfer, efficiency is of an even bigger concern in a wireless power transfer environment. As a result, when attempting to provide power to one or more charging devices, there is a need for methods and apparatuses for communicating between a transmitter and a receiver to determine the power requirements of the receiver.